FIG. 1 is an electrical schematic drawing depicting a prior-art arrangement 10 of an (RF) excited gas discharge laser. The arrangement includes an RF power supply 12, an impedance matching network 14 and a sealed laser housing 16 including discharge-electrodes 28 and 30 and a laser gas mixture. An RF feed-through 22 (in electrical schematic form) for conducting RF power into the laser housing is outlined by a dashed line, as the feed-through from its function can be considered as being both within and without the laser enclosure. The feed-through has a center conductor 24 passing coaxially through a feed-through body 26 which is grounded. The center conductor is the live or “hot” conductor and is connected within the laser enclosure to the hot electrode 28. The feed-through body is connected to the ground electrode 30 and is grounded to the enclosure. All enclosures are RF shielded and grounded.
In an RF power supply the output from a plurality of RF amplifiers is connected via coaxial leads 18 to a common point 19. The RF power is transmitted from point 19 into the impedance matching network via short coaxial structure 20. The ratio of the diameter of a center conductor of the coaxial structure and the inside diameter of the outer conductor are chosen such that the coaxial structure has an impedance as Zp to match the output impedance of the power supply. The impedance matching network includes two L-shaped networks, one thereof including an inductor L1 and a capacitor C1, the other including an inductor L2 and capacitor C2. The L1-C1 network increases the impedance from ZP at structure 20 to ZN at point 21 where the inductor and capacitor are connected. The L2-C2 network matches impedance ZN to the load impedance of the laser ZL.
Certain problems were encountered in configuring and using an example of an arrangement such as arrangement 10 for a laser having an average power output of about 1 kilowatt (kW) or greater necessitating an RF power input of about 10 kW or greater. The RF feed-through selected was of a type described in U.S. patent application Ser. No. 12/069,939 (U.S. Pre-Grant publication No. 2008/0205473), filed Feb. 14, 2008 assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated by reference. This feed-through had previously been successfully used for a CO2 laser having an average output power up 500 Watts (W).
One problem encountered in the higher power laser was that it was difficult to realize practical values for the inductor L2, capacitor C1 and especially for capacitor C2, to match to the low impedance of the laser discharge impedance ZL, and also provide components small enough to fit within an RF shielded enclosure of convenient dimensions. Another problem was that large RF currents flowing within the electrically hot central conductor of the feed-through generated excessive heating in the feed-through which caused problems in using rubber of indium vacuum seals. Yet another problem was that corona discharge occurred between connections from the feed-through to the electrodes within the laser housing. At an RF power level of 20 kW, arcing occurred between metal plates of capacitor C2. There is need to solve these problems in order to provide reliable sealed-off CO2 lasers having an average power of 1 kW or greater.